Abstract
Leguminous plants can establish mutualistic symbiosis with soil bacteria called Rhizobia; the interaction leads to the development of nitrogen-fixing root nodules. The formation of root nodules involves the activities of endogenous phytohormone, such as ethylene, cytokinin and auxin (IAA). The latter seems to have a different role in signaling for the initiation of determinate and indeterminate nodules. Sinorhizobium meliloti induces indeterminate nodules in Medicago species, while Rhizobium leguminosarum is able to form determinate nodules in Phaseolus vulgaris. We have engineered S. meliloti and R. leguminosarum by introducing an auxin-synthesising chimeric operon generating AUX strains, which allows to increase the amount of IAA produced by Rhizobia within bacteroids in the nodules. The aim of our research is to compare the effects of increased levels of IAA in bacteroids on nodulation in determinate and indeterminate legumes. Our results show two different behaviours: Medicago species develop an higher number of nodules when inoculated with AUX strain, while P. vulgaris, nodulated by Aux strain or wild type Rhizobia, forms the same number of root nodules.